The present invention relates generally to data acquisition and, more particularly, to a method and apparatus of increasing the sampling rate used for data MR acquisition using ensemble sampling techniques.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, orxe2x80x9clongitudinal magnetizationxe2x80x9d, Mz, may be rotated, orxe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Generally, the MR signal resulting from the net transverse magnetic moment is demodulated to generate a band-limited MR signal. During sampling of the band-limited MR signal by a data acquisition system, a gradient coil assembly produces a readout gradient. The data acquisition system then generates an MR data set which is ultimately used to reconstruct an image using one of many well known reconstruction techniques.
A number of factors influence sampling rate which can be classified into four distinct categories: (1) analog front end; (2) input signal characteristics; (3) hardware; and (4) analog-to-digital parameters. Factors associated with the analog front end include the sensitivities of the demodulator and the anti-aliasing filter as well as the sample-hold circuitry. Input signal characteristics include the frequency of the input signal, its amplitude and bandwidth, and the noise of the signal. Hardware implemented with the system also may affect the sampling rate, i.e., number of bits, the maximum sampling rate, and the sensitivity of the analog-to-digital converter. Special characteristics of the A/D converter may also affect the sampling rate. The clock frequency, linearity, and operating temperature of the A/D converter can each affect the sampling rate of a signal.
Several techniques have been developed to increase the sampling rate used for MR data acquisition. One known technique uses xe2x80x9cquadrature samplingxe2x80x9d to receivexe2x80x9cinputxe2x80x9d signals from which images may be created. Quadrature sampling involves separating an input signal into two channels by multiplying the original input signal by cos(xcfx89) to form an in-phase (I-channel) and by sin(xcfx89) to form the quadrature-phase (Q-channel). In accordance with this technique, a local oscillator produces an in-phase signal and a quadrature signal. The phase of the end-phase signal is then shifted by 90xc2x0. The shifted end-phase signal and the quadrature signal are then mixed and further processed to generate an output signal having a desired sampling rate and a desired bandwidth that may be used by an MRI system control. Sampling the I and Q channels in parallel effectively doubles the effective sampling rate of the original input signal. This technique and other known techniques, however, fail to adequately increase the sampling rate and bandwidth that are needed for several MR data acquisition applications.
It would therefore be desirable to have a system and method capable of creating a single digital channel with increased bandwidth and subsequent improved sampling rate for MR data acquisition.
The present invention provides a system and method of increasing the sampling rate for MR data acquisition overcoming the aforementioned drawbacks. By implementing ensemble sampling techniques, the present invention provides higher data sampling rates that are useful for several MR data acquisition applications including Echo Planar Imaging, Functional Magnetic Resonance Imaging, and Sensitivity Encoding Imaging (SENSE) techniques. By multiplying an MR signal by a series of pure sinusoids having the same frequency but shifted by an incremental phase, the MR signal may be separated into a number of channels which can be sampled at lower rates by analog-to-digital converters. The output from the converters may then be reconstructed using one of a number of interpolation techniques to create a single digital channel with increased bandwidth. The single channel with increased bandwidth may then be used to acquire MR data with an improved sampling rate.
Therefore, in accordance with one aspect of the present invention, a method of increasing the sampling rate used for MR data acquisition is provided and includes acquiring MR data and combining the MR data with an ensemble function. The method further includes separating the MR data and ensemble function into a number of channels and sampling and converting data from the number of channels to digitize the data. The method also includes reconstructing the digitized data to create a single channel of data with increased bandwidth.
In accordance with a further aspect of the present invention, a computer readable storage medium is provided having stored thereon a computer program representing a set of instructions that when executed by a computer causes a transceiver to detect an MR data signal and demodulate the MR data signal to generate a band-limited analog MR data signal. The computer is further programmed to combine the band-limited analog MR data signal with a number of ensemble functions and input the signal resulting therefrom into a number of analog-to-digital converters. The computer program then causes the computer to detect analog-to-digital converter output and generate a single digital channel with increased bandwidth from the output.
In accordance with another aspect of the present invention, an MRI apparatus to acquire MR data with increased sampling is disclosed. The apparatus includes an MRI system having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system and an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly to acquire MR images. The MRI apparatus also includes a computer programmed to input an MR data signal to a demodulator configured to demodulate the MR data signal into a band-limited MR signal. The computer is further programmed to generate a set of ensemble signal channels by combining the band-limited MR signal by a plurality of ensemble functions and convert the ensemble signal channels to a number of discrete ensemble digital channels. The number of discrete ensemble digital channels is then formed into a single discrete digital channel having an increased bandwidth.
In accordance with yet another aspect of the present invention, a method of increasing the sampling rate used for MR data acquisition is provided and includes generating a polarizing magnetic field across a field of view. An RF signal is then applied to produce transverse magnetization in a region of interest of the magnetic field. The method further includes detecting an MR signal resulting from the transverse magnetization and demodulating the signal to generate a band-limited MR signal. The band-limited MR signal is then multiplied by a plurality of ensemble functions. Thereafter, the multiplied band-limited MR signal is then separated into a number of channels whereupon each channel is sampled to generate a single digital channel with increased bandwidth thereby providing an increased sampling rate for data acquisition.
In accordance with yet a further aspect of the present invention, an apparatus for producing an MR data set from an MR signal is provided and comprises a magnet for producing a polarizing magnetic field and an RF coil for producing transverse magnetization in a region of interest of the polarizing field. The apparatus further includes a receiver configured to receive an MR signal resulting from the transverse magnetization as well as a demodulator configured to demodulate the signal to generate a band-limited MR signal. A gradient coil assembly is provided for producing a readout gradient during sampling of the received MR signal as well as a data acquisition system for sampling the band-limited MR signal and generating an MR data set having a sampling rate more than twice that of the band-limited MR signal. The apparatus further includes a pulse generation system configured to control the synchronized operation of the RF coil, gradient coil assembly, and the data acquisition system. A processing system is also provided for reconstructing an image from the data set.